In lumbering operations it becomes necessary, after trees are cut, to move the felled logs from the area where they have fallen to a location near a road where they can be loaded and carried away. One way in which this has been done is to move into the area with a tractor-like vehicle and simply drag the logs to the road. This is often an unacceptable method because of damage to the terrain and to young trees caused by the vehicle and the dragging of the logs. In many areas the terrain is too rough for such vehicles to operate successfully.
A more acceptable technique and one which has been used in many forms for years involves the use of logging systems with skylines extending from a yarder machine over many hundreds of feet to a tail block with a carriage movable back and forth on the skyline between the yarder and the tail block for transporting the logs to a road near the yarder. The yarder itself is typically a large and heavy vehicle with a prime mover of 300 to 500 horsepower or more, a tower with fairleader sheaves, various drums with cable of different sizes, and clutches and brakes for controlling the reeling of the cable or wire rope off or on the drums and through the various fairleader sheaves. A typical yarder may be capable of being set up to operate with several kinds of rigging arrangements. One such arrangement is called grapple yarding and is described in detail in U.S. Pat. No. Re. 27621 (common assignee). In this system a "running skyline" is rigged from a skyline drum through a fairleader sheave at the top of the tower out to a tail stock and back to a stationary terminal on the far side of the carriage from the yarder, the carriage having one or two sheaves riding on the skyline. A separate mainline has both ends connected to mainline drums on the yarder and passes around a sheave on the carriage. A tag line attached to one of the mainline halves passes over a load sheave on the carriage and is connected to trip a grapple which is suspended from the carriage to cause it to close to pick up a turn of logs or to open to drop the logs. Normally, the skyline is slackened to cause the grapple to lower toward the logs and is tightened to lift the load before the carriage is caused to move toward the yarder.
A somewhat similar arrangement is described in U.S. Pat. No. 3,531,000 (common assignee) but is used for operation with chokers. In this sytem, a crew in the field attaches a choker cable to a turn of logs, and the carriage is lowered by slackening the skyline so that the choker cable may be attached to the carriage or to a line suspended from the carriage. In either of these arrangements, the yarder must include the drums mentioned above, and such drums are sized differently because the skyline and mainline cables are usually of different size. Some rigging arrangements require three different sizes of cable. The cables are typically wound twelve or more layers deep on the drums which means that the effective diameters of the drums vary as they are wound and unwound with accompanying changes in forces and speeds connected with cable movement. To deal with these changes the yarder operator is required to operate various brakes and clutches which absorb substantial energy and which also require considerable operator skill. The winding of many layers of cable on the mainline drums also results in some wear on the cables due to crushing and abrasion.